Upon checking with my friend who is a registered speech pathologist, this is quite alarming. Baby talking is good, though. However, for kids of that age bracket, they should already know how to express with sentences. There are underlying problems which may be the cause. Are the parents always out and do not have the time to bond with the kid? Are they allowing the kid to be drained in front of gadgets?
My friend suggested to bring your niece to a doctor to monitor the over-all development of the child. Engaging her also to outdoor activities which allows her to interact with other kids can also help her develop speech.
Magbasa pa